Swelling and transport properties of hydrophobic polyelectrolyte gels

Abstract

This thesis project is concerned with the development of characterization of a class of hydrophobic gels that can undergo a reversible volume transition between swollen hydrophilic and collapsed hydrophobic states induced by solution pH. The gels employed are amphiphilic in nature and are based on a hydrophobic, water immiscible n-alkyl ester of methacrylic acid (nAMA) and a hydrophilic, ionizable monomer N,N-dimethylaminoethyl methacrylate (DMA). These gels remain collapsed and hydrophobic at, and above, neutral pH but generally undergo an abrupt swelling transition induced by lowering the pH into the acidic region. Increasing gel hydrophobicity significantly reduces the extent of equilibrium swelling and the pH of the swelling transition. The effects of solution pH, ionic strength and ionic valence on equilibrium swelling are shown to perturb the osmotic balance of the gels, and the observed changes in swelling are qualitatively predicted by the Donnan equilibrium theory, although the theory fails when applied quantitatively. The kinetics of gel water sorption and swelling from the dry state are generally non-Fickian, are believed to proceed via a moving front mechanism and are strongly influenced by the chemical nature of the ions in solution. Swelling and deswelling processes from the swollen state are rapid and completely reversible when pH remains below the pH of the swelling transition. When the transition pH is traversed, kinetics become more complicated and generally slow down considerably. Potential mechanisms for the slowing phenomena are investigated.